Tidy up the glossary mixins

Glossaries were:

- partially styled by `#content>#description-list` (got rid of that)
- mixins were defined in both `#content` and `#part` (moved them all into `#part`)
- reduced the arguments to the mixins (they only need `@part`)

I also regenerated the CSS for all the books (since they all use slots thanks to `ccap-base-skeleton.less`)